1. Field of the Invention
The present invention relates to a control method for a four-wheel drive vehicle.
2. Description of the Related Art
In the case of turning a corner having a small turning radius in a four-wheel drive mode of a four-wheel drive vehicle in a low to medium vehicle speed range, a difference in rotational speed due to a difference in turning radius is generated between the front and rear wheels of the vehicle, causing a tight corner braking phenomenon. As the prior art for eliminating such a tight corner braking phenomenon, front and rear wheels driving devices are disclosed in Japanese Patent Publication Nos. Hei 7-61779 and Hei 7-64219.
The front and rear wheels driving devices disclosed in these publications have such a structure that a speed increasing device is provided between main drive wheels and auxiliary drive wheels to thereby adjust an average rotational speed of the auxiliary drive wheels to an average rotational speed of the main drive wheels. This speed increasing device includes a lockup clutch and a speed increasing clutch, which are selectively switched between ON and OFF states to thereby obtain a lockup condition where the average rotational speed of the main drive wheels and the average rotational speed of the auxiliary drive wheels are substantially equal to each other or a speed increase condition where the average rotational speed of the auxiliary drive wheels is greater than the average rotational speed of the main drive wheels.
Particularly in the front and rear wheels driving device disclosed in Japanese Patent Publication No. 7-61779, a torque distribution ratio between right and left rear wheels are controlled according to a vehicle speed and a steering angle so that the rear wheel torque is larger than the front wheel torque and the turning outer wheel torque is larger than the turning inner wheel torque. In this front and rear wheels driving device, the auxiliary drive wheels are increased in rotational speed by the speed increasing device in turning a corner having a small turning radius in the four-wheel drive mode, thereby preventing the tight corner braking phenomenon.
In a control method for a four-wheel drive vehicle, changing the control method in braking the vehicle has been proposed in many documents. For example, Japanese Patent No. 2534732 discloses a drive force control method for a four-wheel drive vehicle wherein when a brake pedal is depressed, the drive force distribution ratio between front and rear wheels is controlled so that the drive force distribution ratio of the rear wheels to the front wheels is reduced. Japanese Patent Laid-open No. Sho 58-56923 discloses a two-wheel/four-wheel drive mode switch control device for performing a control method for a four-wheel drive vehicle so that the drive mode can be switched between a two-wheel drive mode and a four-wheel drive mode according to a running condition, wherein when slip occurs in the two-wheel drive mode, the two-wheel drive mode is automatically switched to the four-wheel drive mode, and when a brake signal is generated in braking the vehicle, the four-wheel drive mode is switched to the two-wheel drive mode.
Japanese Patent Application No. 2004-105026 filed by the present applicant discloses a control method for a four-wheel drive vehicle having a speed increasing mechanism capable of being switched between a same speed condition and an increasing speed condition of the rear wheels with respect to the front wheels, wherein the same speed condition can be switched to the increasing speed condition according to a running condition, and when braking is detected, the increasing speed condition is switched to the same speed condition.
The reason for such changing of the control upon depression of the brake pedal is that it is generally considered that the operator depresses only the brake pedal and intends to decelerate the vehicle. However, there is a case that both the brake pedal and the accelerator pedal may be simultaneously depressed in the following circumstances.
(1) In the case of starting a vehicle having an automatic transmission on a slope
(2) In the case of adjusting acceleration during turning by depressing the brake pedal simultaneously with depression of the accelerator pedal
If braking is detected in either the case (1) or (2) mentioned above to change the control in a four-wheel drive vehicle, there is a possibility that the behavior of the vehicle may become unstable or the effect of four-wheel drive may be reduced. Therefore, if both the brake pedal and the accelerator pedal are simultaneously depressed and the accelerator opening is greater than or equal to a certain value, it is preferable to regard the operational condition of the vehicle as a driving condition, so that the control can be made stable.